Dual-Loop High Speed Voltage Balancing Control for High Frequency Four-level GaN Totem-Pole PFC With Small Flying Capacitors

Flying capacitor multilevel (FCML) converters offer many advantages such as low switch device voltage stress, small current ripples and reduced inductor voltage swing. One critical problem of FCML converters is capacitor voltage balancing. This problem could cause poor system performance and high device voltage stress. Various voltage balancing methods have been developed. Some of these methods are for DC/DC and suffers from low loop bandwidth, which are not applicable for AC/DC applications. Control strategies developed for AC system either requires unique carrier sequence or designed for systems with large flying capacitors in the range of several hundred micro-farad, which would increase complexity for implementation and be less appropriate for system with small value flying capacitors. A voltage balancing control for FCML Totem-Pole PFC using small flying capacitors is proposed in this paper. The proposed method utilizes the net charge of each flying capacitor as the control variable for voltage balancing. Besides, a loop with proportional controller is combined with the net charge control loop to form a dual loop and to improve thew performance. In addition, switching states and associated charging status is also considered in designing this PS-PWM based dual loop system by adjusting duty ratio of each device separately. The proposed dual loop is verified on a 4-level GaN-based 100kHz PFC using small value ceramic flying capacitors. Simulation and experimental results both show a very good balancing performance.